Electrical fuse



Oct. 3l, 1944. H. A. TRIPLETT v ELECTRICAL FUSE 6 Sheets-Sheet l Filed Dec. 20, 1934 7.65 7 2 Z/m/o/O 933%@ L... iv

0d. 3l, 1944. H, A TR|PLETT 2,361,666-

I ELECTRICAL FUSE n Filed Dec. 20, 1934 6 Sheets-Sheet 2 CURRENT IIY AM PERES i /aol 0.1 1.0

TIME nv 55cm/D5 To BLOW )f4/5E JH'N Oct. 3l, 1944. H A TRlPLETT 2,361,666

ELECTRICAL FUSE Filed Dec. 20, 1934 6 Sheets-Sheet 3 i oct. 31, 1944. H. A. TmPLETT 2,361,666 y ELECTRICAL FUSE Inf/@Zinn Oct. 31, 1944. H A TRlPLETT 2,361,666

ELECTRICAL FUSE Patented Oct. 31, 1944 ELECTRICAL FUSE Hugh A. Triplett, Wilmette, Ill., assigner to Schweitzer a.: Conrad, Inc., Chicago, Ill., a corporation of Delaware Application December 20, 1934, Serial No. 758,372

(Cl. Zoli- 117) 34 Claims.

My invention relates to electrical fuses.

While the preferred form herein disclosed rey ticular use or capacity but intend to include all4 phases of the invention, wherever useful.

According to present practice, distribution systems usually operating at from 5000 volts to 15,000 volts are protected by fuse devices of Various construction and variously designated. Because of the large number required in a system of any extent, the item of initial expense` and maintenance is a paramount consideration. The air break fuse has chiefly for this reason been widely adopted. Due to the location of these devices remote from-the central station busses and with the reactance of the lines intervening, the amount of current that must be interrupted is limited to values which make air break fuses operating upon the expulsion principle fairly satisfactory. These fuses are disposed in or on various forms of mountings.

The air break fuse of the expulsion type comprises essentially a tube of insulation with a fusible link threaded therethrough. The fusing of the link upon overload is intended to create suddenly such a volume of vapors and gases as to expel the terminal or terminals from the end or ends of the tube and thereby to interrupt the circuit.

The typical commercial embodiment of the expulsion fuse early took the form of a tube of insulation with ferrules upon the ends, and a closure for one end with the fusible portion of the link disposed adjacent said closed end. Great numbers of devices of this character have been employed, and are still in commercial use.

It was early recognized by my assignee that the simple expulsion fuse, while, in general, it operated fairly satisfactorily on relatively heavy overloads, came into difficulty on low overloads, as the expulsive force was not great enough to expel the link and cause separation of the terminals, and'interruption. Destruction of the device resulted. Fuses of large current carrying capacity are less subject to this particular diiiiculty because the blowing of a large capacity link releases enough energy to create a satisfactory expulsive effect. ticularly useful for low amperage fuses.

Another difficulty with fuses of this type hasv been the uncertainty as to time-current characteristics, particularly on low amperage fuses.

A. further uncertainty, resided in the uncertain length of arcing time of fuses as heretofore constructed.

The 'prior United States patent to Conrad, No. 1,466,423, largely overcame these difficulties by placing the fuse link under spring tension and surrounding the fusible element with a cork. Em-

My present invention is par ployment of spring tension was practicable because a. method of relieving the fusible element from stress and possible injury by the spring load had been satisfactorily worked out, i. e., the use of the so-called strain wire. Upon the occurrence of overload great enough to cause fusing of the fusible element, the terminals were promptly separated far enough to insure expulsive action and interruption of the current flow.

I have conceived the expulsion fuse as being, of necessity, a dual range device. According to my conception of the ideal expulsion fuse, it must have the capacity for operating in one fashion upon low overloads, and in another fashion upon high overloads, but these two modes must be so coordinated as to be mutually helpful or to overlap in producing the desired result. In brief, the two modes of operation must work in the same general direction, so that in the intermediate range when both actions occur, they may overlap and work to the common end; The device of my present invention is designed to Ido this.

In considering various forms of fuse devices upon the market, I have conceived the possibility of incorporating within the fuse link itself the principle of operation of the aforesaid Conrad United States Patent No. 1,466,423, so that the resultant device employing such link will have the desired plural range action and suitable time-current characteristics, shortarcing time, etc.

One of the primary objects of my invention is to provide a fuse link for use in fuse holders of various forms or for various mountings to constitute the same as plural range fuse devices with suitable predeterminable timeurrent characteristics and arcing time for each desired capacity.

For the accomplishment of this object and other objects, I have devised a form of link, which I term a universal link, having such structural form and adaptability as to permit its utilization in substantially all forms of commercial fuse devices of the general class here under consideration.

In carrying into effect the broad concepts of my invention, I have devised numerous improvements both to the end of operating characteristics and structural refinements and simplification.

One of the objects of these improvements is toprovide a fuse link containing within itself the necessary means for insuring the desired characteristics of operation, independently of the particular mounting in which it is placed. L Another object of the invention is to provide an improved form of fusible element for the link. said element having a high temperature metal which is relatively free from corrosion, and has long life, accuracy in time-current characteristics, and the ability to stand momentary surges without deterioration.

A further object of the invention is to provide for connecting a pair of fuse terminals by composite conductor means which includes a high conductivity metal sheath, the ends of the composite conductor means being held in the fuse terminals principally by frictional engagement.

A further object is to provide a fusible element which shall be suitable for withstanding the tension required in spring-loaded or weight-loaded cutouts, auch as automatic dropouts orreclosing fuses, and yet be satisfactory for use in cutouts putting no tension on the link.

Another object of the invention is to provide a link provided with terminal portions adapting it to service in all or substantially all modern designs of cutouts.

A further object is the provision of links for devices of the type hereinabove referred to with accurate time-current characteristics and suitable operating ability in the entire range of elements from one ampere 'to 150 amperes. The links of my present invention are particularly accurate within the range of from 1 to 10 amperes, where the greatest accuracy is desirable.

Accuracy in blowing time of fuse links would be of little or no importance if heavy short-circuit currents were possible at the points where most cutouts are installed. The fact is that on most systems, and particularly rural systems, heavy short-circuit currents are the exception rather than the rule. Therefore, this means that most cutouts sold are subjected to an interrupting duty which is out on the long time blowing end of the time-current curve. Out on the low current end of the curve, reliable accuracy in blowing time and current is essential if the company insists on good operating results. By good results, I mean that the accuracy of the fuse link is such that its blowing can be so coordinated with the circuit breaker at the head of the feeder, and so coordinated with any other fuse in sequence on the line, that when a fault occurs on the load side of the cutout, the fuse link will isolate the fault in time to prevent these other fuses or breakers from opening and thus dropping the load in adjacent territory. This requires, in a fuse link, both accuracy in time-current characteristics and reliable short arcing time. No matter how accurate the melting of the link, long and indefinite arcing time will undo the benefit of such accuracy by causing other devices in sequence to operate.

Another object of the present invention is to provide a link responsive to fault current of the order of one to three or four amperes without sacrificing either thermal capacity to withstand lightning transients or the tensile strength of the wire used as the fusible section. Whereas a simple wire fusible section would have to be of hairlike dimensions, and thereforewould be mechanically weak with respect to spring tension, and entirely unsuited to be installed in a cutout employing a tensioning spring, the fuse of the present invention permits the use of a heavier wire which is entirely adequate in tensile strength. The link of the present invention will melt and part at the same overload or fault current which would melt the wire of hairlike proportions. 'I'he fuse of the present invention will, therefore, with stand the spring tension which is found in all of the cutouts of my assignee and in the cutouts of the new dropout type which are coming into the market.

In carrying out this phase of the invention I employ a pair of wires, preferably of nickel-chromium alloy, having hooked engagement with 'II each other. The wires are of adequate mechani cal strength to resist the necessary spring tension but the hooked connection will readily yield under the spring tension winch is provided by straightening out the hooked or bent parts of one or both of the wires. By the provision of a molded bead of a low melting point metal or alloy which encases or embeds the hooked engagement, the pulling apart of the hooked engagement is prevented until the bead softens sufiiciently to permit the end of the hook to swing about through the body of the low melting point metal and straighten out suiliciently to pull clear of the cooperating anchorage.

A further vobject of this invention is to provide a fuse link which has a very sharp melting and parting point on the temperature curve, that is a link which does not soften and stretch and thus reduce in cross-section as the temperature closely approaches the definite melting point. In the present construction the bead of tin or other low melting point material either allows the hooked engagement to pull apart or it will not permit the bent portion of the hook to straighten out at all. The present fuse link, therefore, does not result in a progressive lengthening of the fusible section, as would be the case if the two wires were soldered together or otherwise connected through the body of the low melting point metal.

Another object of the invention is to provide a fuse link with a novel and distinctive form of time-current characteristic curve, as illustrated in exaggerated form in Figure 9. A fuse having such a characteristic curve, that is, with the short and medium time blowing points elevated, permits the momentary surges or transients, such as lightning induced current, to pass through the device without melting of the bead. There is, therefore, less cumulative effect of repeated surges which might otherwise cause the link to melt and part at normal load current. In that part of the curve corresponding to long time blowing there is a relatively sharp dip beyond the duration time of surges and transients. Being at the long time end of the curve, and being a substantial drop in the curve, it serves as a Drotection against detrimental thermal effect in the transformer to which this link is applied as a protection. It is known that long continued overload of the order of 50% or possibly 100% on the transformer results in destruction of the insulation. It is therefore desirable to have the long time end of the curve as low as possible with respect to the normal load current of the transformer while the medium and short blowing time of the curve should be higher with respect to the normal load current of the transformer. It is desirable, therefore, that these two sections of the curve rbe joined by a section which is as nearly vertical as it is possible to provide. This object is particularly desirable for fuses of the order of one to two ampere ratings which blow on currents ranging from one to three or four amperes.

In addition to the foregoing objects there are other and numerous objects of the present invention which will be apparent from the following detailed description and the appended claims.

Now in order to acquaint those skilled in the art with the manner of constructing and operating a device in accordance with my invention I shall described, in connection with the accompanying drawings, several specific embodiments of the invention.

In the drawings, the same or similar reference Figure 27 is an enlarged sectional view of the characters designate the same or similar parts, throughout. l

Figure 1 is a side elevational view of a low amperage fuse link of the order oione to two amperes. the specic fuse link shown is a tw ampere link;

Figure 2 is a longitudinal axial section through a fuse of larger amperage, for example, three to ten amperes;

Figure 2-A is a detail sectional view taken along the line 2-A-2-A of Figure 2;

Figure 3 is a vertical, longitudinal section through the fuse link of Figure 1, taken on the line 3-3;

Figure 4 is a view of the fuse link and terminals taken at right angles to the view of Figure 3;

Figure 5 is an enlarged fragmentary view showing the hooked connection between the fuse wires of the structures shown in Figures 1, 3 and 4;

Figure 6 is a view similar to Figure 5 of a modification; f

Figure 7 is a similar view of another modication, showing the hooked connection in section;

Figure 8 is a section of the structure shown inr Figure 'l taken at right angles to Figure "1;

Figure 9 is a diagram illustrating the type of time-current characteristic curve which is secured by the fuse of Figure l;

Figure 10 is a time current characteristic curve showing thefrelation between current in amperes and time in seconds to blow fuses of the present invention plotted on a logarithmic sca-le;

Figure 1l is a side view of an adapter for the upper terminal end of the link of my invention;

Figure 12 is a similar View of the same, with the top broken oiv to provide an upwardly facing notch;

Figure 13 is a side view of another form of upper terminal fitting;

Figure 14 is an edge view of the same;

Figure 15 shows the fitting of Figure 13 with the button head broken off and a notch broken out of the side edge to provide a laterally opening notch;

Figure 16 is a side view of a form of fuse link having iiexible leads at each end;

Figure 17 is a side elevational view of a clamp fitting for releasably attaching one end of the fuse link of my invention to a continuous conductor;

Figure 18 is a section taken on the line I8-I8 of Figure 17, showing the tting and link applied to a continuous conductor;

Figure 19 shows the fuse link of my invention installed in one form of commercial mounting;

Figure 20 shows the fuse link of my invention mounted in a dropout bok type of mounting;

Figure 2l shows the fuse link of my invention mounted in a standard commercial form of box employing no loading upon the fuse link;

Figure 22 shows an open dropout fuse mounting in which my fuse link is utilized;

Figure 23 is another commercial form of dropout fuse mounting which places a spring and gravity load upon the fuse link, and may employ the fusevof my invention;

Figure 24 is a further commercial form of open fuse mounting in which a springA loading is -externally applied;

Figure 25 shows the fuse link of my invention installed in a so-called ejector cutout;

Figure 26 shows the fuse link of my invention applied directly to secondary distribution conductors and forming a link between such conductors and a drop wire;

radiation shield and coupling member employed in the structure of Figure 2;

Figure 28 is a cross sectional view taken on the line 23-23 of Figure 27;

Figure 29 is a side view of a modifiedl form of hooked connection;

Figure 30 is a similar view of the same in releasing position; y

Figure 31 is a longitudinal section of a fuse of higher current carrying value; and

Figures 32, 33, and 34 are fragmentary longitudinal sectional views of mcdiiled forms of the fuse link construction.

Referring ilrst to Figures 1, 2, 3 and 6 I have shown a fuse link I which necessarily involves three essentials. First. a terminal portion 2, such as is suitable for attachment to one electrie terminal and support, a fusible section 3 (Fig. 3), and a. flexible lead and terminal portion 4 for making connection with another electric conductor with which the link is to be connected in series. The portions 2 and 4 are pref*- erably adjustable to meet various conditions of l mounting, as I shall describe more in detail. The link is preferably enclosed vin a fuse tube such as the fuse tube 5 shown in Figure 25, and as shown in the prior Conrad United States Patent, 1,818,382. The flexible lead of terminal por-- tion 4 comprises a stranded copper or the like connector the outer end of which, as shown at B, is tinned to hold the strands together and which is provided also with a collar member l, this collar member I having an intermediate portion 8 which is pinched or attened upon the body of the lead 4 and stamped with the ampere rating, as indicated at S. and provided with. a flared or conical wedging contact I0 for engagement in a cooperating stationary wedging con tact shown in the copending application of Alan Ramsey, Serial No. 671.660, and as indicated at I2 in Figure 25.

The upper terminal 2 comprises a cylindrical stud member I3 (see Fig. 4), the cuter end of which is threaded at I4. The intermediate portion comprises a ilange I5which may represent the original diameter of the stock from which the stud is cut. as by a screw machine. Adiacent the flange i5 there is a groove I6 and beneath the same a knurled portion I 1. The '"nner end of the stud I3 comprises a tubular socket portion I8 into which is inserted the upper end of the Wire I9 of the fusible link 3. The tubular portion I8 is then flattened or pinched upon the end of the wire to make good electrical andv mechanical connection with said wire I9.

The threaded stud I3 is adaptable to a wide variety of situations for making connection between the link and a suitable terminal. either stationary or movable, as the case may be. For connection with a stationary mounting wherein an annular seat and a cooperating cap or clamp is employed. as indicated at reference numeral 25 in Figure 22, a flanged head 26 is provided. This ilanged head comprises a tubular sleeve or shank internally threaded to cooperate with` the threads I4 of the stud I3 and having its upper end flanged or flared out as indicated at 2l as ry a spinning operation. Where the annuler eat is of greater extent than would be suitably rn.- gaged by the flange 21. a dished washer 2. is provided, this washer being passed over'the link from the opposite end and resting unde.` the fiange 21. The end of the stud I3 is substantially ilush with the top of the rilange 21 and provides a good clamping surface additional to the top of the conical flange 21.

The wires I9 and 20 have hooked engagement at 22, shown in detail in Figure 5. A molded bead 23 of a low melting point alloy, such, for example, as solder, is molded on the hooked engagement and embeds the ends of the wires I3 and where they make engagement. The lower end of the wire 23 is folded back on itself like the upper end of the wire I3 and is disposed in a tubular fitting 23, the upper end of which provides a socket 30 which is flattened upon the end of the wire 23 to grip it mechanically and to make connection electrically. The lower end of the fitting 2S, which is preferably made of a piece of copper tubing or the like, is similarly pinched or flattened upon the upper end 43 of the flexible lead 4.

This fitting 29 therefore forms a coupling or junction member between the lower end of the fusible element 3 and the lead 4. It also forms an anchorage for the tension spring 33, which has its upper coil flattened against the flat sides of the pinched socket 30 and is prevented from disengagement by the shoulders 34 of the flattened part of the socket 32, since the flattened portions of the sockets 30 and 32 are so disposed that their planes are substantially at right angles to each other.

The spring 33 has its opposite end anchored directly to the flattened collar 35 which is pinched upon the flexible lead 4. Indirectly, the spring is anchored to the cap or plate 36 through the medium of the fitting 1, which has its adjacent end 37 in engagement with the plate or cap 36.

The margins of the plate or cap 36 are so formed as to center the plate or cap upon the end of a fiber tube 38. The upper end of the tube 38 is securely bound to the upper terminal stud I3 in axial alignment therewith and rigid for both rotary or axial motion with respect thereto. This engagement is secured preferably by wetting the end of the nber tube 33, pushing it over the knurled portion I1, and then shrinking the end into the groove under the flange I5. Such shrinkage may be facilitated by pinching.

I do not wish to be limited to this particular method of fastening these parts, as other means of securing the fiber tube in place may be provided as, for example, by shrinking a metallic band 38', Figures 2 and 2A upon the upper end thereof, or by suitable cementing, or otherwise.

The spring 33 thereby places the link 3 under a predetermined tension which, in the present instance, is of the order of seven pounds. The link shown in Figures 1, 3, 4 and 5, has the fusible element formed as shown in Figures 3, 4 and 5, and has the spring anchorage at the lower end of the tube 38 constructed in the same manner as shown at the lower end of Figure 2.

The link shown in Figure 2 employs a fusible element 3a which is slightly different in construction from that shown in Figures 1, 3, 4 and 5, as this link is designed for carrying capacities of from 3 to 10 amperes in the specific embodiment shown.

There is a definite coordination between the minimum spring tension applied to the fusible element and the character of the said fusible element, as will now appear.

In the S. 8: C. cutouts of the type shown in aforesaid United States Patent 1,818,382, the tension of the spring which tends to separate the terminals of the fusible link ranges from 10 to 12 lbs. This amount of tension is necessary in view of the unavoidable mass of the terminal and cable assembly which is required to extend the rating of any particular cutout to include links of higher ampere rating, say up to 60 amperes,

Now inasmuch as the present link is designed and adapted to be inserted in cutouts of that type as well as in other forms of mountings, fuse tubes and the like, it is desirable that the fusible element be capable of withstanding spring tension of that order, that is, from 10 to 12 lbs. I have provided the hooked engagement 22 and the soft metal bead 23 as a means for permitting the employment of wires I9 and 20, which are ample in cross section and mechanical strength to withstand spring loads or other loads of from 10 to 12 lbs. The wires I3 and 20 will not fuse at from 1 to 3 or 4 amperes, which is the definite overload value at which the fuse shall open, but thehooked engagement, combined with the minimum spring tension, is designed to do this under the control of melting of the molded metal bead 23. That is to say, the wires I3 and 20 must maintain good mechanical strength throughout the life of the device and must maintain substantially unchanged cross-section. These wires are made of metal which does not corrode, deteriorate due to oxidation, or is not otherwise attacked by atmospheric ingredients or components. Preferably, I employ an alloy of nickel and chromium which appears on the market under the trade name of Chromel A. Obviously, I do not wish to be limited to this specific material, as any other metal which has noble characteristics may be employed.

The critical point of the construction resides in the straightening out, or release, or pulling apart of the hooked connection.

I am aware that it is old to have parts soldered together or held together by a solder or soft metal rivet or the like, but those devices are subject to change in characteristics due to an approach to the melting point and a recession therefrom. In the 1 ampere fuse size I employ wires for 1 ampere rating of the order of 0.016 inch in diameter, and for 2 ampere rating of the order of 0.018 inch in diameter of the aforesaid material, and where the total length of the fusible link between the terminals I3 and 30 may be of the order of 1 inch.

The hooked engagement 22 is preferably that shown in detail in Figure 5, wherein the wire I9 has a closed loop or eye 42 formed upon the end thereof, this eye being, for example, circular and formed by bending the stock adjacent the end into a circular annular loop, with the end of the wire, as at 43, brought parallel with the main body thereof, and welded thereto. The wire 23 likewise has a loop 44 of similar configuration, but having its end 45 free. A bead of low melting point metal, for example, tin, solder or the like, is then cast about this hooked engagement, as shown in Figure 5, and this device thereupon forms a mechanical relay controlled by temperature. Obviously, instead of forming one loop completely closed by welding the end 43 as shown in Figure 5, the ends of both wires may be left open, as shown in Figure 6. In the sizes of wire shown without the metal bead 23 cast in place, a tension of only 3 to 4 pounds is sufllcient to straighten out the hooked engagement suillciently to allow the wires I3 and 20 to pull apart. But with the bead in place the hooked engagement is stronger aseaoeo than the main part of the wires i3 and 2l, until the bead 23 ls softened by heat. When the bead 23 is softened by heat generated in the wires I3 and 2li by current flow, the wires I 3 and 20 can be pulled apart by straightening out the hooked engagement at 7 pounds or less.

'The conductivity of the bead 23 surrounding the hooked engagement is so great that this part of the fusible element does not become sumciently elevated in temperature by current iiow but heat is generated in the intermediate parts of the wires I and 20, that is, between the bead 23 and the terminals I8 and 30, and the heat is conveyed to the bead y23, bringing it up to a temperature where it softens` suillciently to permit the tension of the spring to separate the parts. The wires I8 and will not soften sutllciently under heat to cause any weakening thereof short of approximately 800 to 900 C., but the metal bead 23 will soften between 2'00 and 300 C. There is, therefore, a wide diilerence in the eiective temperatures of the two parts of the fusible link. y

It vis to be understood that the amount of spring tension which is fplaced upon the fusible link affects the time-current characteristic of the link. That is to say, if the spring tensionis excessive, obviously it will break the link without any current flow. If the current flow is great enough to aiect the temperature of the link it wil1 affect'the mechanical strength, even though the link be but a straight wire. If the wire alone be considered, no appreciable weakening in mechanical strength will be caused by current ilow until the wire reaches a critical temperature, whereupon the spring tension will have a marked effect upon the time-current characteristic. The fusible element shown in Figures l, 3, 4 and 5 is a. compound device in which it is intended, as hereinabove stated, to secure an oiset in the time-current characteristic ilow which is idealized in Figure 9.

In that curve I have shown only the general contours and not absolute values. The continuous full line curve shows the characteristics desired. The dot-and-dash curves indicate continuations of the two independent curves which are combined by my invention. The part of the curve running from P to M is a part of a curve for one capacity of fuse. The part of the full line curve running from N to Q is part of the curve of a lower capacity link. Assume that the line M-N is a substantially vertical line and that this is placed at substantially the 60 second value, for example, it will be assumed that for values of current greatly in excess of the normal load current the blowing time is relatively shortI but a marked increase of current is required if the fuse is to blow in less than 60 seconds. Within this range the wire I8 or 20 itself melts. Within the range N-Q the bead melts and releases the link. It is desirable that this should occur at as small an increase above normal load current as it is possible to secure in order that apparatus on the line, such as transformers, will not be subjected, for any appreciable period of time, to values of current flow substantially in excess of the normal rating. y Y l In practice it is not possible to have as sharp a junction as is indicated by the vertical line M-N. In Figure 10.1 have shown the shape of actual curves as plotted from tests made on fuses of this character. The lower two curves, marked Nos. 1 and 2, are curves representing the performance of one and two ampere fuses constructed in accordance with the disclosure of Figures 1, a, 4 and 5. n wm be observed that these curves are of the general shape'indicated by Figure 9,

the relatively sharp dips indicated in the region M'N showing the pronounced eilect which is produced by the introduction of the bead and hook construction heretofore described.

As above indicated, the hook and bead principle may be embodied in quite diil'erent forms. For example, Vin Figure 5, the hooked free end 43 may be considered as a lever hinged to the main shank portion oi' the wire 20 with a predetermined friction at the pivot as representing the force required to` swing the free end 4I into an unhooking position. Obviously, the principle would not be changed by having the free end 45 pivoted to the main shank portion without friction. YThis is illustrated in Figures 29 and 30. In other words, the end 45, which constitutes a lever, might be pivoted to the main shank by an actualpivot joint giving a known leverage of relatively high value. The real resistance to unhooking of the connection of these two wires residesin the molded bead 23. In order to move to an unhooking position the free end 45 must swing sidewise through the metal of the bead.

To permit this the metal of the bead must beV soft enough to allow the arm 45 to swing therethrough. This mode of operation is entirely different from prior devices, in which a straight wire or a wire bent into zig-zag or coiled form is embedded in a bead, since according to that construction, spring tension allows a slow creepage or lengthening of the link to occur, thereby changing its rating and rendering the device unreliable. In the present case, even if the free lever-like end 45 should begin to swing and then be halted, no appreciable lengthening of the wire 20 or the link as a whole could be detected. Hence, the characteristic of the link would not be changed, and that would be true of any position of the wire 45 short of allowing an actual unhooking or slipping through the eye 42.

In Figure 6, I have indicated open hooks on both the wire I9 and the wire 20to give the hooked connection 22. In practice it is not necessary or desirable to have more than one of the wires provided with an open hook, inasmuch as there is a tendency for the bead 23 to be rotated and, as the bead is oblong, it might come into a position in the ber tube 38 which would interfere with its free rotation and expulsion. Furthermore, contact of the bead 23 with the inside of the fiber tube is not desirable, as it tends to alter the characteristics of theldevice.

In Figures 7 and 8, I have shown the eye 42 as provided with a small roller 4l formed of a definite exteriorlv diameter, so. as to control accurately the pivotal point about which the free end 45 will rotate. By thus providing a definite radius and conforming the end of the wire 20 to that radius, asnug and certain iit may be obtained, showing very definitely the yielding point of the link. Also, it is to be understood that instead of the two wires directly hooking with each other they may hook with an intermediate link, such as shown in Figures 29 and 30, which provides either the hook portion or the eye portion.

Whereas fuses of the low order of rating of one or two amperes present the greatest diiliculty in providing a universal fuse which may be subjected to the various loads, springs, weight, and' otherwise, which mountings now on the market provide, fuses as low as from 3 to 10 amperes likewise present a distinct problem. In Figure 2, I have shown in detail the construction of a 3-ampere fuse employing, like the fuse shown in Figure l, a predetermined spring tension to insure the operation of the fuse in accordance with its desired time-current rating. The upper terminal member I3 is the same as that shown in Figures 3 and 4, and comprises the upper threaded stud portion I4, the intermediate fianged and grooved portion I5-I6, the knurled portion I1, and the tubular collapsible socket. portion I8 which is adapted to receive the corrugated upper end of the fusible wire 48 and to be pinched ilat thereupon. The tting 23a, shown in more detail in Figures 2'? and 28, comprises two parts, namely, and 32, which are identical with the parts of the fitting 2B shown in Figures 3 and 4, but in addition this fitting has an integrally formed metal radiating shield 48 which surrounds the major part of the wire 48 which constitutes the link 3a.

The enlarged tubular portion 48 surrounds loosely the wire 48 and lies in axial alignment out of contact with the inner walls of the ilber tube 38 in which it is contained. The small gap between the shield 49 and the upper terminal I8 prevents shunting of the fuse wire 48 but permits static or steep wave front discharges to occur from the terminal I8 to the said radiation shield 49 without traversing the wire 48. Thus, sharp static overloads which might otherwise cause blowing of the fuse unnecessarily, are ineffective to cause operation. The provision of the shield 48 forms no part of the present invention. It is disclosed and claimed in the copending application of Lindell, Serial No. 70,280, led March 23, 1936, and assigned to the assignee of this application.

The wire 48 is preferably nickel-chromium alloy, secured upon the market under the trade name of Chromel A. This is the same material of which the wires I8 and 20 are made. Tests show that this nickel-chromium wire retains considerable mechanical strength at elevated temperatures (even at red heat), but that a rather abrupt weakening occurs at temperatures which have been computed to be about 800 to 900 C. The melting point of these wires is approximately 1400 C.

The effect of these properties of the nickelchromium wire is to produce a certain relationship between the flowing current and the mechanical tension, which has been utilized to render these links practically independent of external mechanical forces, within certain limits. I have made tests to determine the eiect of spring tension upon the blowing time of a link. 'I'hese tests show that a direct relationship exists. For example, the current required to blow a fuse wire of a diameter of .0204" in 100 seconds (this ap` proximates the minimum fusing current for the particular terminal arrangement employed during those tests), amounted to 12.5 amperes, when no tension is applied. 'Ihe current, however, decreases rapidly with increased spring tension up to about four pounds, which means that rupture occurs at temperatures below the melting point progressively approaching the critical range of from 800 to 900 C. as the tension approaches 4 lbs. for this particular size of wire, corresponding to a current of approximately 7 amperes. Any further increase in tension results in a gradual but slight decrease in the blowing current until the ultimate whole strength of the wire is reached, whereupon the blowing current abruptly drops to zero. That is to say, there is a purely mechanical failure. The characteristic curves of the fuses of the structure shown in Figure 2, employing the nickel-chromium fuse wire 48, are shown in Figure 10 at curves numbered 8, 8, and I8. This arrangement of the spring 88 so that the fusible element, for example the wire 48, is non-cumulatively stressed within certain limits forms no part of the present invention but is disclosed and claimed in the hereinbefore mentioned copending application of Lindell.

In Figure 31, I have shown the fuse wire 4I of nickel-chromium alloy as shunted by a conductor 5I, preferably of silver. Other low resistance metals or alloys might be employed instead. In the present instance, the silver wire 5I, being of much lower resistance electrically as well as thermally, constitutes a Droportionin shunt carrying the main part of the current and melting ahead of the nickel-chromium wire 4l, which is now employed as a strain wire. Due to the much higher conductivity of the silver wire 5I, as compared to the strain wire 48, contact between them is permissible so long as it is not appreciably varied. Hence the silver wire may be wrapped upon the strain wire, or it may be employed as a hollow tube through and beyond which the strain wire extends. Both the strain wire and the silver wire or tube are preferably mechanically gripped in tubular terminals which are pinched tight upon them.

The fuse construction embodying the silver tubular fuse element is shown in Figure 33 of the drawings. As there illustrated a fusible element in the form of a silver tube 8l is provided surrounding the strain wire 48, the ends of the latter extending beyond the ends of the former, being of zigzag shape and secured in the pinched socket portions I8 and 80', of the terminals I8 and 28 w-respectively. The ends of the silver tube 8| are secured in position, when the socket portions I8 and 88' are pinched thereon. As indicated, depressions are created by a suitable tool, such as a prick punch, in the pinched socket portions I8 and 88' for increasing the conductivity between them and the strain wire 48 and the fusible element II The upper convolution of the spring 38 is flattened about the shoulders 84 that are formed by the flattened socket 82 in which the upper end 48 of the exible lead 4 is secured.

- The tubular fusible element 8|' may be spaced from the strain wire 48, as shown, which construction corresponds to that illustrated in Figure 38 in which the fuse wire 5I is'spaced from the strain wire 48 or it may be in engagement therewith throughout its length, as indicated at 5I" in Figure 34, corresponding to the construction shown in Figure 31 in which the fuse wire 5I is wound tightly on the strain wire 48.

When the silver conductor forms a tube, it may serve as a corona shield serving that function o! the shield 48 of Figure 2. Also it runs cooler because of the increased area. The high conductivity wire 8l may be coiled loosely about and out of contact with the strain wire 48 as shown in Figure 32.

The arrangement of the flexible lead and preloading spring in Figure 31 is the same as that shown in Figure 2.

In order to adapt the fuse links of my invention to mounting in various devices such as are now employed, I have provided the ttings or accessories illustrated in Figures 11 to 18.

Figure 11'shows a fitting 81, preferably made oi a drawn tubular piece of metal such as a copper tube, having a threaded socket 88 at the lower end for cooperating with the threaded stud I4 of the fuse of Figure 1 or of Figure 2. The upper end of the iltting is expanded or spun out to form a conical head 21 substantially identical with the head 21 shown in Figures 1 and 2. An intermediate flattened portion 64 is formed by pinching the walls of the tube together. A tongue 85 is sheared out oi the upper end of the ilattened metal portion 84, so that the head 21 may be broken oi and the tongue 88 removed, to leave a notch 88, as shown in Figure 12, where a mounting around a bolt or pin is required for connecting the link at its upper end to a corresponding terminal.

A conical washer 28, as shown in Figures 2 and 13, may be employed, where the head is retained for mounting upon an annular seat, as will be explained later.

The terminal 61 shown in Figures 13 and 15 is similar to that shown in Figure 11, except that in this instance a tongue such as 88 is struck laterally in the ilattened portion 84, so that when the head is broken off and the tongue broken out, the terminal appears as shown in Figure 15, with the notch 69 in the side edge of the flattened portion 84. This adapts the terminal for connection in certain binding post arrangements in mountings now in use. The use of partially sheared portions, which might be termed knockouts, permits the utilization of the metal for electrical and heat conductivity with ready removability. This, I believe, is broadly new.

Figure l4 shows an edge view of the structure shown in Figures 11 and 13.

In Figure 16 I have shown a link constructed in accordance with my invention, in which the upper iitting corresponding to the terminal I3 of Figures 1 to 4, instead of having a threaded stud I4 has a socket 18 in which is received and mechanically held, as by pinching thereupon, the flexible lead 12. A similar ilexible lead 13, corresponding in general to the lead 4 of Figures 1 to 4, is connected to the lower end of the fusible link within the fiber tube 38. This form of link is particularly adapted to mountings in which the fuse vtube is open at both ends.

In Figures 17, 18 and 26, I have shown a form of mounting in which the device of Figure 1 or Figure 2 is adapted to be employed as a complete operative fuse device in connecting a house drop conductor to a line or distribution conductor. This device comprises a connector 14 adapted t0 make connection with a continuous conductor such as 15, as shown in Figures 18 and 26. The secondary distribution conductors 15, 16 and 11, shown in Figure 26, are supported upon a secondary rack 18, providing suitable insulators for each conductor, the rack being mounted upon a pole 19 or the like. The drop 88 comprises, in this case, a pair of concentric conductors having leads 82 and 83 which are adapted to be connected to the secondary distribution conductors 15 and 11. The concentric pair forming the drop 88 includes a supporting strand, which may be a suitable wire cable 84, anchored upon the pole 19 through an insulating eye 85 having a threaded shank screwed into the pole. The connector 14 comprises a U-shaped body, the base of which, 88, is hexagonal in form and the parallel arms of which are' threaded as indicated at 81 to receive a clamping nut 88. A transverse bar 88 having a shank 98 extending inside of the nut 88 is of a width to substantially iili the slot 92, so that when the nut 88 is threaded down to clamp the bar 88 upon the conductor 18, the arms 81 will not be able to escape from the nut 88. A shank 88 has the edges thereof riveted over the top edge of the nut 88, so that the bar 89 and the nut 88 will remain together, even when they are removed from the tting 14 to admit the continuous secondary conductor 15. The conductor is `preferably insulated, but it has the insulation l removed in order to accommodate the clamp 14.

The clamp 14 has a threaded` opening axially through the bottom hexagon portion 86 to receive the shank I4 of the upper terminal I8 o! the link shown in Figure 1 or Figure 2. The ilange I5 is screwed up tightly against the bottom or base of the hexagon portion 88 to make good electrical connection and to support the link with the fiber tube 38 in horizontal position. Theexible lead 4 is connected to the corresponding conductor of the drop 88, as shown in Figure 26. Within the ber tube 38 is disposed the spring 33, and inthe event of overload in the circuit including the conductors 82 and 88, the fusible link is,fused and the lower terminal, to which the spring 33 is attached, retracts the said movable terminal ysuch as 28a. The spring recoil, with the addition of the gases freed or formed upon blowing of the fuse, expels the terminal 29a from the tube and throws the wire 82, with the connected link 4, to the right as viewed in Figure 26, into the clear. The conductors 82 and 83 may be so disposed and formed as to facilitate this action. From this style of mounting it can be seen that the fuse link is a complete fuse device within itself, capable of maintaining its predetermined characteristics quite independently of any aid from an additional fuse tube or the mounting in which it may be disposed.

The general purpose to which this form of fuse link may be applied is illustrated in Figures 19 to 25, inclusive. Figure 25 has heretofore been mentioned, and it comprises a standard device on the market manufactured by my assignee, and is designated as the S. & C. ejector cutout. It comprises a tubular porcelainl or like insulating housing 93 within which are mounted annular terminals (not shown) for making connection with the corresponding annular terminals on the fuse tube 94. The lower ferrule on the fuse tube 94 carries an arm 86 with a conical wedge-like socket to receive the conical wedge-like member I8 shown in Figure 2, the lead 4 below the said member I8 being cut off when the device is installed. The upper end of the unit I has the threaded stud I4 threaded into a threaded socket 91 which, in turn, is connected to the spring 58. Upon operation of the device under excessive current ow the spring 58 pulls the stud member I3 with the connected fiber tube 38 upwardly, and

the spring 33 throws the lower terminal, such as 29 or 29a, downwardly. At the same time, the gases of the arc tend to assist in the expulsion of the said lower terminal, the spring 33, the flexible lead portion 48, and the plate 38.

In the operation of blowing of a device such as shown in Figure 25, employing the fuse link of Figures 1, 3 and 4, there are two ranges of operation wherein diierent modes of action are se.- cured, all to a common end of accurate predetermined operation. Assume that the excess current is in the lower ranges wherein the wires I9 and 28 are heated by the ilow of current but not to the point where these wires lose their mechanical strength, that is, in the region at the right of the hump in the curves indicated at Nif-N in Figure 10. The wires I and 20 transmit heat to the low melting point bead 23 and, if the current is maintained for a period of time which will permit the bead to become softened to the point of allowing the hook wire to swing about through the body of the bead and become unhooked, the fuse thereupon operates to interrupt the circuit.

Inasmuch as in the device of Figure 25 the spring 50 has a pull of the order of 10 to 12 lbs.,

whereas the spring 33 has a pull of the order of" 7 lbs., the total stress upon the fusible link is only 10 or 12 lbs. Hence, no substantial deviation from the type of curve shown in Figure 10 is occasioned by this form of mounting.

In the mounting shown in Figure 25 the fuse tube is carried by the handle 38 and may be removed for renewal. y

For higher values of current flow above the hump indicated at M'N, the flow of current tends to soften the wires I9 and 2|! to such an extent that they yield mechanically beforesuiiicient heat is transmitted to the bead 23 to cause softening of the same, and fusion of the bead is not depended upon to secure opening of the circuit. In this event the wires I9 or 20 pulls apart, due to the spring tension, and the circuit is opened as above described, with the combined action of the springs and the gases formed upon blowing of the fusible link.

The ber tube 38 is not intended to be expelled with the movable fuse terminal, as it is anchored ilrmly to the stud member I3. It serves the useful purpose of confining the gases and assisting in the expulsive eiIect, particularly on the lower current values. Also, it protects the inside of the permanent fuse tube such as 94 from injury by the arc. Also, it serves to assist in evolving deionizing gases, as it appears that the decomposition of the ber by the arc is helpful in the action of deionizing the arc by driving gases evolved from the walls of the tube endwise of the arc to sweep out the ionized product and metal vapors. 'Ihe sleeve 38, while it is preferably made of plain gray ber, may be specially constructed to assist in the evolution of gases as by impregnating the same with a liquid or solid arc extinguishing medium, as disclosed in the patent to Ringwald, Reissue No. 19,097.

In Figure 24 I have shown the link of my invention employed in a mounting having a fuse tube 93, the upper ferrule of which has a knurled cap |00 and an annular seat for receiving the button head such as is shown in Figure 1 or Figure 3, the flexible lead 4 in this instance is attached by a clasp |03 attached to a leaf spring |04 anchored at |05 to the lower ferrule |06. Upon melting of the fuse the leaf spring |04 flips the lead wire 4 and connected lower terminal out of the lower end of the fuse tube 99. In this construction the leaf spring |04 supplies spring teni sion for any link which is mounted in the device.

hinged cover I0, the cover being adapted to carry u the fuse tube ||2 cooperating with spring terminals ||3 and ||4 mounted within the box. The fuse tube I I2 has an upper ferrule with a threaded cap member ||6 and an annular seat for clamping the button head such as is shown in Figures 1 and 2 to make connection with said upper ferrule. At the lower end the flexible lead 4 is led out through the lower end oi the open tube as indicated at I6 and the end of the lead is brought under the annular knurled nut II1 and there clamped to the surface of the lower ferrule. In v this mounting there is no additional spring tension to that which is imposed by the spring 33 within the inner fiber tube 38 of the link. The operation of the device is substantially as heretofore described, that is to say, the predetermined tension of the spring 33 acting upon the fusible element causes the same to be parted when the proper current value is reached, the resultant parting assisting in expelli. g the flexible lead and the lower terminal and connected parts.

Obviously, 'in any of these mountings, if the current flow is so heavy as to result in a violent blowing of the fuse, the inner tube or sleeve, which is the sleeve 38 of the link, may be shattered, but even in such operation the inner surface of the main or outer sleeve or tube ||2 is protected in large measure.

In Figure 19 I have shown the link of my invention as employed in 'a cutout box ||0 having a lid IIS. The fuse tube |20 carried upon the lid member and the ferrules |22 and |23 are provided with projecting contact springs formed of pairs of wires for engaging stationary fiat studs |24 and |25 mounted on the bottom of the box member IIB. The fuse tube |20 employs a retractile spring such as that indicated at 50 in Figure 25, with a flexible lead |26 connecting the threaded socket |21 and the threaded plug |20 which closes the upper end oi the ferrule |22. The threaded socket |21 receives the threaded stud of the stud member I3 and the flexible lead 4 is brought out of the lower open end of the tube and brought back to a clamp-terminal |29 connected with the lower ferrule |23. In this construction the spring 50 applies the main spr.ng tension which, as in the mounting shown in Figure 25, may be in excess of that of the spring 33 of the element or link I. The mounting shown in Figure 19 is disclosed and claimed lin the copending application of Allan Ramsey, Serial No. 559,846, issued as Patent No. 2,091,452. The tension of the main retractile spring 60, working through the link I, is sustained on the lower open end of the tube where the nexible lead 4 passes over the edge or lip of the same.

In Figure 20 I have shown a dropout type of box which is shown in detail and claimed in the copending application of Allan Ramsey, Serial No. 734,262. In this device the lid |30 which carries the fuse tube |32 is held in closed position on the box |33 by the integrity of the link I in the fuse tube |32. The fuse tube |32 has an upper ferrule |34 with a threaded cap |35 for clamping upon an annular seat the button head such as shown in Figures 1 and 2 to make connection with the upper terminal. The fuse tube |32 is mounted in a fitting |36 with respect to Awhich the fuse tube may have a sliding motion under the influence of the spring to move the fuse tube downwardly to disengage the pin |31 from the latch spring |33 that connects with the upper stationary terminal |39. The lower end of the fuse tube bears` upon a. pivoted link |40 hinged at |42 to the tting |36. A stationary same time the fuse tubev |55 descends and as the button head escapes from under the hump |60 bracket |43 and the ferrule |34 serve to mount the fuse tube |32 on the lid |30. A cam-shaped clutch jaw |44 grips the lower end of the iiexible lead 4 against the corresponding jaw on the link |40. The tendency of the spring' in compression is to thrust the fuse tube |32 down against the link |40 and this reaction is taken up by the tension in the flexible lead 4 and thence in the entire link. Upon melting of the fusible element the fuse tube |32 drops down as the link |40 is no longer sustained by tensionv of the link the pin |31 is unlatched from the spring latch |38 and the door drops open. l

In Figure'22 I have indicated a mounting of the fuse tube |45 to secure a dropout effect of the said fuse tube |45 from the mounting. This mounting comprises an upper stationary bracket |46 comprising a hood containing suitable contacts and including a spring-mounted pin |41 which is adapted to be engaged by the hook |48 formedron the movable sleeve 49 of the upper ferrule |50. A compression spring between the sleeve |49 and the stationary ferrule member |58 tends to throw the hook |48 upwardly. The sleeve |49 has a threaded cap 25 for clamping the button head of a suitable spring mounting. A tension spring 50, shunted by a suitable flexible conductor |26 carries at its lower end a threaded coupling |21 as in Figures 19 and 25 and the stud member I3 of Figures l and 2 is threaded into said fitting |21, whereby the upper portonof the link I is mounted within the lower end of the fuse tube |45, the exible lead 4 is carried down to the lower end of the extension on the ferrule |52 and connected under a binding post |53. When the fusible link melts or yields the spring 50 pulls the stud and tube 38 upwardly, releasing the sleeve |49 and permitting the hook |48 to travel upwardly and detach itself from the pin |41. Thereupon the spring contacts which bear upon the sleeve |49, and gravity, throw the fuse sleeve |45 in a clockwise direction about the trunnion |54, taking the fuse tube |45 out of the circuit.

In Figure 23 I have shown another form of mounting which is on the market. The fuse tube |55 in this structure has an upper ferrule |56 and a lower ferrule |51. The upper ferrule |56 has an opening through the same permitting the sleeve 38 of my link to be passed therethrough, allowing the button head 21 to project. A linger I 58 projects upwardly from the upper ferrule .|56 and is designed to pass between a pair of spring rods |59 forming the upper terminal. spring rods press downwardly, both to make contact with the button head 21 and to throw the fuse tube |55 into open position when the fuse element of my link melts. The lower ferrule |51 has an arm |62 pivoted to a bell crank lever |63, one end of the lever being held against the lower end of the fuse tube |55 by the exible lead 4 which is fastened under the binding post |60. The bell crank lever |63 has a hook about the trunnion |64, which trunnion is mounted on the lower stationary terminal of the mounting. The downward thrust of the spring rods |59 is sustained by the link which holds the arm |65 of the bell crank lever |63 against the lower end of the fuse tube |55. When the link is ruptured by melting of the fusible element the downward thrust of the spring rods |59 and the weight of the fuse tube |55 tends to cause a rotation of the bell crank lever |63 in a clockwise direction about the trunnion |64. This action pulls the flexible lead 4 outwardly in the tube |55. At the These in the spring rods'i59, the fuse-tube |55 ldrops out of the circuit. l,

From the above illustrations it can be seen that the fuse link of my invention is interchangeable in a wide variety of mountings .which now employ fuse links of the prior art.

The above illustrated mountings are but typical examples of mountings in which the fuse of my sired mode of operation. 'I'his spring tension is great enough for the very low ampere 'fuses such as one or two amperes, to overcome the hooked engagement ofthe fuse wires when the bead melts. Mere melting of the bead does not release the hookedconnection of the` Wires, and this spring tension is therefore necessary to secure opening of the circuit for current values which are lower than those which would actually melt the wires such as i9 and 20. In this manner it is possible to secure a very delicate response to low current values while employing parts of ample mechanical strength and dimensions.

I believe that the hook and bead principle is broadly new. I am .aware that it has been attempted to hold parts together by solder, wax` and various other expedients, but in all of the structures with which I am familiar, there is' no positive mechanical engagement such as I have obtained by the use of the hook and eye, independently of the bead or solder.

lAnother feature of my invention above described is the provision of attachments and iittings for making the fuse applicable to a wide variety of mountings. A particularly important feature is the employment of the threaded stud with interchangeable terminal parts, whereby the link may be applied toa wide variety of terminal fittings.

Furthermore, the link of my invention is a complete fuse within itself, and may be so employed,

as shown, for example, in Figure 26. In this form it is suitable for lower voltages, such as to 220. Obviously, by changing the proportion of parts the voltages may be carried higher. The fuse employed in Figures 1'7-18`and 26 may omit the springv where the fuse rating is great enough to provide the expulsive action.

Numerous other advantages in manufacture and operation will be apparent to those skilled in the art.

I do not intend to be limited to the detalls shown and described.

I claim:

1. In combination with a fuse link comprising a first fuse terminal member. a second fuse terminal member, and a fusible element connecting said terminal members, a fiber tube having one end bound upon said first member. said member having a threaded stud projecting out of the end of the tube, a flexible lead connected to the second member, and a iitting having a flanged head at one end, a at intermediate portion, and a. tubular threaded socket at the other end threaded upon said stud. i

2. An adapter for fuse links of the class described, comprising a hollow tubular metal member of a soft metal of high conductivity having a flanged conical head at one end and a tubular internally threaded socket at `the other.

3. An adapter for fuse links of the class described, comprising a hollow tubular member of cuprous metal having a flanged conical head at one end, an intermediate flattened portion, and a tubular internally threaded socket at the other.

4. A fuse link comprising a pair of terminals, a strain wire connecting the terminals, a tubular conductor of high conductivity surrounding the strain wire and interconnecting said terminals, a sleeve of insulation surrounding the strain wire and said tubular conductor, and a spring normally stressed and tending to separate said terminals.

5. The combination of a fuse device comprising a fuse of the tubular expulsion type, clamping means at one end of the tube for rigidly securing it to a selected point on a powerwire, a flexible lead at the opposite end of said tube for connection with a drop wire, and a spring inside of said tube for discharging said flexible lead away from the fuse device when the fuse blows.

6. In a replaceable fuse link a pair of terminals, a tubular conductor interconnecting said terminals, and a strain wire positioned inside of said tubular conductor and also interconnecting said terminals.

7. In a replaceable fuse link a pair of terminals, a fusible conductor in the form of a silver tube interconnecting said terminals, and a strain wire also interconnecting said terminals, said strain wire being disposed Within said silver tube. Y

8. In a fuse device a pair of terminals, and a pair of concentrically arranged fusible conductors disposed in contact with each other and interconnecting said terminals and secured thereto by frictional engagement therewith,v one of said conductors consisting of silver.

9. In a fuse device a pair of relatively massive terminal members, a fusible element in the form of a tube interconnecting said terminal members, and a strain element positioned within said tube and interconnecting said terminal members for relieving said fusible element of stress tending to separate said terminal members.

10. A replaceable fuse link comprising a fuse tube impregnated with an arc extinguishing material which is adapted under the action of an arc to give off a gaseous arc extinguishing medium, a fuse terminal at one end of said fuse tube, a flexible lead extending out of the other end of said fuse tube, a tubular fusible element composed principally of silver interconnecting said fuse terminal and said flexible lead, a tension spring disposed around said flexible lead inside said fuse tube and reacting between the same for extending the gap formed on blowing of said fusible element, and a strain wire inside of said tubular fusible element and interconnecting said fuse terminal and said flexible lead for relieving said fusible element of the stress imposed by said spring.

1l. An adapter for fuse links of the class described. comprising a hollow tubular metal member of high conductivity having a flanged head at one end, an intermediate flattened portion, and a threaded portion at the other end.

12. A fuse link comprising a pair of terminals, a strain wire and interconnecting said terminals connecting said terminals, a tubular conductor of high conductivity surrounding said strain wire. and a sleeve of insulation surrounding said strain wire and said tubular conductor.

13. In a fuse link, a strain wire having corrugated end portions, and a pair of terminals having tubular end portions adapted to be pinched upon said corrugated end portions.

14. In a fuse link, a strain wire having corrugated end portions, and a pair of terminals having tubular end portions, said end portions being generally deformed upon said corrugated end portions and also locally deformed thereon.

15. In a fuse device, a pair of relatively infusible terminal members, a fusible element in the form of a tube secured to and interconnecting said terminal members, a strain element also interconnecting said terminal members for relieving said fusible element of stress tending to separate said terminal members, and means anchoring said strain element to said terminal members independently of said fusible element.

16. In a fuse device, a pair of terminals, and composite fusible conductor means interconnecting said terminals, said composite fusible conductor means including a tubular silver sheath of such cross-sectional area that it conducts a substantial portion of the current flowing between said terminals.

17. In a fuse device, a pair of spaced apart terminals having tubular portions facing each other. and composite fusible conductor means disposed in said tubular portions for interconnecting said terminals, said tubular portions being deformed onto the ends of said composite fusible conductor means therein, said composite fusible 'conductor means including a tubular silver sheath of such cross-sectional area that it conducts a substantial portion of the 'current flowing between said terminals.

18. In a fuse device, a pair of spaced apart terminals having tubular portions facing each other, and composite fusible conductor means disposed in said tubular portions for interconnecting said terminals, said tubular portions being deformed onto the ends of said composite fusible conductor means therein, said composite fusible conductor means including a sheath of metal characterized by being of relatively high conductivity as compared to the remainder of said composite fusible conductor means and of such cross-.sectional area that it conducts a substantial portion of the current flowing between said terminals.

19. A renewable fuse link comprising, a pair of relatively infusible terminals, a flexible lead connected to each of said terminals, a fusible element interconnecting said terminals, spring means biasing said terminals apart and tensioning said fusible element, anda fuse tube surrounding said terminals and said fusible element.

20. A renewable fuse link comprising, a pair of relatively infusible terminals, a flexible lead connected to each of said terminals, a fusible element interconnecting said terminals, and a fuse tube substantially longer than the distance between said terminals and surrounding the same and said fusible element.

21. In a fuse link, a metallic core, a tubular metallic container surrounding said core, a pair of flexible conductors formed by a plurality of strands of wire, and a pair of tubular terminal connectors each having one end adapted to receive said core and container and the other end adapted to receive one of said flexible conductors.

22. In a, fuse link, a metallic core, a tubular silver'container surrounding said-core, a pair of flexible conductors formed by a plurality of strands of wire, a pair of tubular terminal connectors each having one end adapted to receive said core and container and the other end adapted to receive one of said flexible conductors, and a sleeve of insulation surrounding said container and said terminal conductors.

23. In a replaceable fuse link, a pair of terminals, a tubular fusible element electrically interconnecting said terminals, and a strain element mechanically interconnecting said terminals in such manner that tension stress between said terminals is applied directly to said strain element.

24. In a fuse, a pair of spaced apart terminals having tubular end portions extending toward each other, anda tubular fusible element interconnecting said terminals and having its ends telescoped with said tubular end portions, the ends of said tubular fusible element and said tubular end portions being deformed to provide electrical connection therebetween.

25. In combination with a fuse link comprising a first fuse terminal member, a second fuse terminal member, and a fusible element connecting said terminal members, a fuse tube having said first fuse terminal member at one end and surrounding said second fuse terminal member an'd said fusible element, a flexible lead connected to said second fuse terminal, and a'fitting having a. flanged head at one end and a flat intermediate portion and arranged and adapted to be detachably connected-to said first fuse terminal member.

26. A renewable fuse link comprising, a fuse tube, a terminal at one end of said fuse tube, a flexible lead extending out of the other end of said fuse tube, fusible means interconnecting said terminal and said flexible lead, a threaded stud on said terminal, and a fitting having a flanged g head at one end, a flatintermediate portion, and

a tubular socket at the other end threaded upon said stud.

27. In a fuse link, in combination, a terminal fitting, fusible means connected to said terminal fitting, a sleeve of insulation surrounding said fusible means and closely surrounding said terminal fitting, and a retaining ring surrounding said sleeve and holding it in position.

28. In a fuse link, in combination, a terminal fitting having a terminal attaching portion at one end and a fusible element receiving portion at the other end, a fusible element secured to said fusible element receiving portion, a sleeve of insulation surrounding said fusible element in spaced relation and at one end closely surrounding said terminal fitting intermediate said portions. and a retaining ring shrunk around said end of said sleeve and holding it in position.

29. In a fuse link, in combination, a terminal fitting having a terminal attaching portion at one end, a fusible element receiving portion` atthe other end and a knurled portion intermediate said ends; a fusible element secured to said fusible element receiving portion, a sleeve of insulation surrounding said fusible element in spaced relation and at one end disposed around said knurled portion, and a retaining ring shrunk around said one end of said sleeve and holding it in position on said knurled portion.

30` In a fuse device, a pair of spaced apart terminals having tubular portions, and composite fusible conductor means disposed in said tubular portions for interconnecting said terminals, said tubular portions beingV deformed onto the ends of said composite fusible conductor means therein, said composite fusible conductor means including a tubular silver sheath of such crosssectional area that it conducts a substantial portion of the current owing between said terminals.

31. In a fuse device, a pair of spaced apart terminals having tubular portions, and composite fusible conductor means disposed in said tubular portions for interconnecting said terminals, said tubular portions being deformed ontov the ends of said composite fusible conductor means therein, said composite fusible conductor means including a sheath of metal characterized by being of relatively high conductivity as compared to the remainder of said composite fusible conductor means and of such cross-sectional arca that it conducts a substantial portion of the current flowing between said terminals.

32. In a fuse link, two terminals and a fusible element connecting them, and prestressed means maintaining said fusible element under predetermined mechanical tension of such value that said fusible element will part at a predetermined current rating and any increase in tension imposed on said element within the limits of its mechanical strength leaves the current rating thereof substantially unaltered, whereby the current rating of -said element is maintained substantially constant irrespective of increased tension thereof.

33. In a fuse link, an insulating sleeve, a pair of fuse terminals and a fusible' element connecting them,l one of said terminals being mounted on said sleeve and restrained thereby against movement in a direction toward the other terminal, and prestressed means connected to said other terminal and anchored to said sleeve reacting thereon toward said first terminal, said means maintaining said fusible element under predetermined mechanical tension of such value that said fusible element will part at a predetermined current rating and any increase in tension imposed on said element within the limits of its mechanical strength leaves the current rating thereof substantially unaltered, whereby the current rating of said element is maintained substantially constant irrespective of increased tension thereof. 34. In a fuse link, an insulating sleeve, a pair of fuse terminals and a fusible element connecting them, one of said terminals being mounted on said sleeve and restrained thereby against movement in a direction toward the other terminal, and a prestressed spring attached at one end' to said other terminal and having "its other end anchored to said sleeve, said spring reacting up'on said sleeve toward said one terminal and cooperating with said sleeve for maintaining said fusible element under mechanical tension, the stressing of said spring being such that said element is maintained under tension of such value that it will part at a predetermined current 'rating and any increase in tension imposed upon said element within the limits of its mechanical strength leaves the current rating of said element substantially unaltered, whereby the current rating of said element is maintained substantially constant irrespective of increased tension thereof.

HUGH A. 

